Manufacture of single crystal semiconductors



July 21, 1970 s, s -r EI'AL 3,520,810

MANUFACTURE OF SINGLE CRYSTAL SEMICONDUCTORS Filed Jan. 15, 1968 2 Sheets-Sheet 1 FIG.1

Fl G 2 (513) FIG.3

INVENTORS THOMAS S. PLASKETT JERRY M. WOODALL WILLIAM C. WUESTENHOEFER BY ATTORNEY July 21, 1970 5, PLASKETT ETAL 3,520,810

MANUFACTURE OF SINGLE CRYSTAL SEMICONDUCTORS Filed Jan. 15, 1968 2 Sheets-Sheet 2 FIG 5 1200 T TEMP. GRADIENT II00 AT SEED- LIQUID 1000 INTERFACE l/ TEMP. 900 I we) 800 I 100 II 600 I I I l I I l I I CENTIMETERS /GGAS MELT 00 c As RESERVOIR As RESERVOIR Go+GoAs Z an Y/j/l v SEED POSITION 2 POSITION I STRESS F I G 7 (I00) 2 United States Patent Olfice 3,520,810 Patented July 21, 1970 US. Cl. 25262.3 4 Claims ABSTRACT OF THE DISCLOSURE GaAs injection lasers operate more efficiently and yield higher power outputs if single crystals of GaAs having a high degree of crystalline perfection (free of dislocations and defects resulting from chemical inhomogeneities) are used. For operation as an essential element in an injection laser, the gallium to be combined with the arsenic is heavily doped, such that the resulting GaAs crystal has a carrier concentration of approximately 2-4 10 /cc. Previous attempts to grow heavily doped dislocation-free GaAs crystals from a melt by the conventional horizontal Bridgman technique have been un-' successful. While extreme care has been taken to control pressure and temperature conditions during the growth from a melt of single crystals of GaAs, it was not realized that the dislocation density of a crystal depended on its growth orientation. This disclosure teaches that heavily doped crystals of GaAs grown in the 031 direction are essentially free of dislocations, and that such novel teaching can be applied to the growth of any doped compound provided that the stoichiometry of the compound can be varied and that the growth parameters are maintained under severe control. This disclosures also teaches that dopants with distribution coefficients farthest from unity are preferred for growing crystalline perfect doped crystals. It also teaches that higher As overpressure will enhance crystalline perfection. It further teaches that the stoichiometry of doped crystals changes with growth direction.

BACKGROUND OF THE INVENTION It has been ascertained that the power and efiiciency of GaAs injection lasers have been increased by the use of GaAs crystals having a very high degree of crystalline perfection. Improved crystalline perfection has been demonstrated in a detailed examination of crystals grown in various crystal orientations in accordance with conventional horizontal Bridgman growing techniques.

It has been known prior to this invention that undoped GaAs crystals have been grown using the Czochralski technique and such crystals can be grown dislocation-free. However, where GaAs has been doped, dislocation occurs in the final crystal and the dislocations increase in number as the doping gets heavier. The present invention is predicated on the discovery that the presence of dislocation defects in doped crystals was found to be dependent upon crystal growth orientation where growth occurred using the horizontal Bridgman technique. It was found that the highest quality crystals are obtained when the GaAs crystals were doped with tin and grown from a GaAs seed oriented along the 03l direction.

While the present invention produces better single doped crystals by growing on a crystal seed whose face lies in an (031) plane, certain other discoveries have made the invention applicable to a wide variety of compounds. By altering the stoichiometry of the GaAs crystal, the high temperature strength of the crystal increases, thus reducing the tendency to generate dislocation during growth. In doped crystals, i.e., where tin is a dopant, such deviations from stoichiometry occur by varying the growth orientation which, in turn, varies the dopant concentration, hence, stoichiometry, in the liquid at the solid-liquid interface. If the As overpressure in the crystal growing system is maintained constant at the dissociation pressure for undoped stoichiometric GaAs, then dopant variation results in a change in the stoichiometry of the growing crystal. Increasing the tin concentration in the melt will increase the number of gallium vacancies in the crystal, so that the stoichiometry of GaAs is not truly one atom of gallium and one atom of arsenic. This alteration changes the physical properties, i.e., it strengthens the GaAs, and maximum strength occurs if growth takes place along the 031 direction.

In general, any doped compound or doped alloy compound of the zinc blende structure can have its stoichiometry altered by growing in the O31 direction.

Further, any undoped compound may have its high temperature strength increased by the addition of materials to the melt which do not alter the chemical identity of the crystal.

It is an object of this invention to grow dislocationfree single crystals of GaAs.

It is yet another object to grow dislocation-free doped GaAs.

A further object is to make dislocation-free GaAs crystals that can be used in the manufacture of injection lasers.

Yet another object is the adding of a material to the melt so that the only effect is to change the stoichiometry of the crystal.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a completed growth of monocrystalline GaAs in the form of an ignot.

FIG. 2 is a showing of the axis along which said ingots are cut.

FIG. 3 is a showing of a furnace and its contents for growing pure single crystals of doped GaAs.

FIG. 4 is a showing of the quartz boat and its contents.

FIG. 5 is a plot of temperature versus furnace depth during the growth of such crystals.

FIG. 6 is a plot of stress v. strain for a material.

FIG. 7 is a showing of the injection laser made in accordance with the teaching of this invention.

Prior to describing the invention, certain conventions that crystallographers use will be defined. A number commonly called a Miller index surrounded by i.e., indicates a family of planes in a cubic crystal equivalent to the plane in such brackets. A number in parenthese, i.e. (100), indicates a specific plane in a cubic crystal, namely, the 100 plane and no other. The symbol l00 represents a family of directions indicating any and all directions corresponding to cube edges whereas the symbol [100] designates a specific direction, namely, the 100 cube edge. In most of what follows, crystal growth takes plate in the {031} plane along the O31 direction.

DESCRIPTION OF THE PREFERRED EMBODIMENT Apparatus for growing single crystal elements and compounds by the temperature gradient freeze process is set forth in US. Pat. No. 3,242,075 to D. H. Harris, which issued Mar. 22, 1966. In such patent, one method disclosed comprises placing polycrystalline GaAs in a crucible, melting the polycrystalline material in the crucible, then placing the crucible in a tubular furnace which is capable of producing a temperature gradient along its length so that the crucible is hotter at one end than at the other end. As the temperature of the furnace is reduced, and the gradient is shifted, a portion of the material within the crucible located in the cooler portion of the furnace will freeze, causing a solid-liquid interface. When the gradient has shifted to a point below the freez ing point of the material, a single crystal structure is formed. In the Harris patent, it was found that by the judicious use of heat-conducting and heat-insulating shields about the entire length of the crystal, one could maintain the application of substantially linear heat along the entire length of the crystal and thus obtain uniform temperatures at the liquid-solid interface of the crystal as it is being pulled through the furnace. Such uniformity of heat application aided in obtaining growth of nominally uniform single crystals of GaAs. In such Harris patent, however, no mention is made of growing pure single crystals that contain dopants, nor is there any mention of growth along the 031 axis.

In FIG. 1, there is shown a completed ingot 2 of single crystal GaAs grown by conventional methods, i.e., Czochralski technique or horizontal Bridgman technique. The arrow in FIG. 1 indicates the general direction of growth, namely, from left to right of the ingot 2. The ingot 2 was in a heated boat in a furnace, and as the boat was pulled out of the furnace, a portion of the melt solidified, leaving a solid-liquid interface, growth of the single crystals taking place at the interface.

By placing the ingot 2 on a goniometer and taking Laue photographs, using X-rays, of the crystal, such photographs being well known to crystallographers and not essential to the description of the invention shown and described herein, one is able to locate the 031 axis. he ingot 2 is suitably marked with lines 4, 6, 8, etc. that indicate the orientation of the 031 axis and the ingot is cut along these lines.

In FIG. 2 is shown a block that is cut from ingot 2 having two parallel faces that lie in the 031 plane. Growing of single crystals of heavily doped gallium arsenide takes place in one of these faces in a direction that is perpendicular to the face. This block 10 serves as a GaAs seed onto which dislocation-free single crystals of heavily doped GaAs crystals are grown.

As seen in FIGS. 3 and 4, a quartz boat 12, whose interior was sandblasted to prevent wetting by any molten material placed therein, contains gms. of gallium 14 and 1.3 gms. of tin 16. The seed 10 is placed in the left portion of boat 12 with its face (the one lying in the 031 plane) pointing in the direction of the contents of gallium 14 and tin 16 in the boat 12. While it is not absolutely necessary, very favorable results occur if the horizontal plane of the seed is parallel to a {531} plane at the same time that growth takes place along the 031 direction. The {531} plane must be compatible with the condition that the horizontal plane be perpendicular to the {031} growth planes. For a specific growth direction [031], the compatible specific horizontal plane is (51 3). The boat 12 and its contents are placed in a quartz ampoule 18 and gms. of arsenic 20 are placed in a section of ampoule 18, not in contact with boat 12. The ampoule is pumped down to a pressure of 10- Torr., and sealed at a point, such as tip 22; the sealed ampoule 18 is inserted into a furnace 24, similar to the one shown and described in the above noted Harris patent, but any other furnace equivalent in operation to the Harris furnace and which is used for crystal growing can be employed.

As seen in FIG. 5, the quartz ampoule 18 is loaded into a furnace 24 wherein the reservoir of arsenic remains in the relatively cool portion of the furnace and is heated to 600 C. The boat 12 and its contents are placed in position #1 where the Ga and tin are heated to 1240 C., but the seed is kept in the vicinity of 709 C, The ampoule is kept in this first position for approximately 20' minutes, permitting the Ga melt to react to become a GaAs melt and reach equilibrium and also to prevent the seed 10 from being melted by the unsaturated gallium. The arsenic is kept at -1 atmosphere and such pressure is closely controlled by maintaining the temperature of a cold spot in the arsenic region of the ampoule to within i0.5 C., which effectively is the equivalent to a variation in As pressure of about 10.5 atmosphere.

After the 20 minute interval, the ampoule 18 is inserted further into the furnace at the rate of 5 cm. per hour until it rests in position 2 of the FIG. 5. During the last 2.5 cm. of travel, the As reservoir temperature is raised to 616 C. where it is maintained for approximately 16 hours. During this period, the melt is being further saturated with arsenic and a solid-liquid interface I (see FIG. 3) at equilibrium exists between the seed 10 and the melt. After the 2.5 hour period has terminated the melt is allowed to stand for A2 hour to assure proper seeding, at which time the melt is pulled out of the furnace through the temperature gradient shown by curve T. The melt is pulled through this gradient at the rate of -0.85 cm./hr. after which time it is removed from the furnace to cool the room temperature.

For this representative seeding procedure using 40 gms. of gallium, 50 gms. of arsensic, 1.3 gms. of tin, the quartz boat was -14 cm. long and the holder for the gallium arsenide seed 10 was -2.54 cm. long and 1.1 cm. deep. The quartz ampoule 18 was approximately cm. long.

After the GaAs crystal, grown on the {031} plane of seed 10, has been removed from the furnace and is cooled, a p-n junction 26 is formed, using conventional P-type dopants, such as zinc. Then the unit is diced along the (110) planes and cleaved along (110) planes perpendicular to the diced (110) plane. GaAs, when cleaved along its (110) planes, results in perfectly parallel faces. Electrodes 28 and 28 are connected to the two faces that are perpendicular to the planes and the unit of FIG. 7 is now ready to be connected to a suitable source of power, such as battery 30, and be used as an injection laser. Since growth of heavily doped GaAs has taken place in the 031 direction, the GaAs has a particularly low dislocation density, and the latter characteristic makes the GaAs crystal well suited for use in an injection laser, increasing the efficiency and power output of the laser.

For purposes of practicing the invention, a semiconductor is considered heavily doped if the dopant has concentrations between 10 to 10 holes or electrons/cc. While the invention has been described with respect to doped gallium arsenside, it is to 'be understood that crystal growth in the 03l direction is applicable for III-V compounds other than GaAs, such as GaP, or for heavily doped compounds of the II-VI groups, such as PbTe, PbS, ZnS, of the Periodic Table, particularly where the stoichiometry of the melts of the doped compounds are controlled by the vapor pressure of one of the elements of the compound. Phosphorus, arsenic, and sulfur are examples of such components which are used to control melt stoichiometry.

Furthermore, the addition of a noncontaminating component to the liquid during crystal growing should effectively change the stoichiometry of the crystal. A noncontaminating component is one which goes into the solid crystal at a level that is less than 10 atoms/cc. and thus does not alter the electrical properties of the completed solid crystal. Also, by increasing the As overpressure in the crystal growing phase, one may alter the stoichiometry in the desired direction.

In FIG. 6 is shown a typical plot of stress 'rV. strain 6 for a material. As stress is applied to a material, the latter will elongate, and so long as the yield point P is not reached, when the strain is removed, the material returns to its original unstressed position at the ori in of the TV. 6 plot. However, should the stress be applied beyond point P, the elastic limit of the material, the latter is permanently deformed. By changing the stoichiometry of the doped GaAs by growing the doped crystal along the 03l axis, the strain v. stress curve is now depicted by the dotted line. In eifect, the high temperature strength of the crystal during growth is increased. So long as growth direction is that the distribution coefficient (the stresses are uniaxial, the benefits accuring from off stoichiometric growth and judicious choice of growth axis are obtained.

The reason for choosing the 03l direction as the growth direction is that the distribution coefiicient (the ratio of the tin concentration in the solid to that in the liquid) is dependent on the crystal growth direction. From crystal growth theory, it is knOWn that if the distribution coeflicient is less than unity, then the liquid layer ahead of the advancing solid-liquid interface is enriched in tin (or any other dopant). The farther the distribution coetficient is from unity the greater is the concentration of tin in this layer, and this occurs in the 031 direction.

If the As overpressure in the crystal growing system is maintained at the dissociation pressure of undoped stoichiometric GaAs, then the addition of tin causes the liquid to increase in As concentration and thus changes the stoichiometry of the growing crystal. The greater the tin concentration in this layer the further the deviation from stoichiometry and the stronger the crystal. The maximum strengths occur near the 031 direction.

The crystalline perfection of any doped compound with one component volatile can be enhanced if the axis of growth is judiciously chosen. The correct axis is determined by knowing the dependence of the distribution coefiicient with growth orientation. The axis with the distribution coefiicient farthest from unity is required. For the zinc blende structure the 031 is the desirable direction.

It has been shown that dislocation density is related to orientation of crystal growth, with the lowest disclocation density occurring for crystals grown in the -031 direction. Other orientations during growth influence high temperature plasticity of the crystal. The amount of alteration in the ratio between Ga and As due to the presence of tin at the solid-liquid interface during growth of the crystal appears to be dependent on crystallographic orientation. Thus the impurity or dopant concentration profile immediately ahead of the advancing solid-liquid interface is a good measure of the off stoichiometry being produced in the final frozen crystal, and such ofi stoichiometry strengthens the crystal and makes it less susceptible to dislocation damage at high temperatures. Thus, preferred growth directions for compound semiconductors that are heavily doped can be more readily determined.

Throughout the above discussion, it should be clear that along the 031 direction, means substantially along that direction and the invention can be practiced even if the growth orientation deviates approximately five degrees around an 031 axis.

What is claimed is:

1. A method of growing a tin doped single crystal of GaAs by the horizontal Bridgman technique comprising:

providing a single crystal seed of GaAs,

contacting the seed to a melt of GaAs so as to provide a solid-liquid interface between said seed and melt, adding tin to said melt to vary the stoichiometry of the grown crystal, and

growing a GaAs single crystal from said melt along the 03l direction of said single crystal seed. 2. A method for growing a tin doped single crystal of GaAs compound by the horizontal Bridgman technique comprising:

providing a single, crystal seed of GaAs, cutting said seed along the (031) plane, contacting said seed at such (031) plane to a melt of GaAs to provide a liquid-solid interface,

providing a dopant of tin for said melt during crystal growth wherein the distribution coefiicient of said dopant is less than unity, and growing a single crystal of GaAs along the 03l axis of said single crystal seed.

3. In a method for growing a tin doped single crystal of GaAs compound by the horizontal Bridgman technique starting from a single crystal seed of GaAs, the improvement consisting of growing a GaAs single crystal from a melt of tin containing GaAs off-stoichiometrically along the 031 axis of said crystal seed.

4. A GaAs injection laser wherein a component of said laser is GaAs single crystal containing 10 to 10 atoms of tin per cc. of GaAs, grown from a tin containing GaAs melt along the 031 direction of a single crystal seed of GaAs.

References Cited UNITED STATES PATENTS 3,245,847 4/1966 Pizzarello 25262.3 X

TOBIAS E. LEVOW, Primary Examiner I. COOPER, Assistant Examiner U.S. c1. X.R. 

